Inkjet printers work by spraying ink at a sheet of paper or other print medium to create images or text. Inkjet printers are capable of producing high quality print approaching that produced by laser printers. Inkjet printers are generally less expensive than laser printers, but can also be considerably slower.
To produce words or pictures contained in data received by a printer from a host computer or network, the inkjet printer squirts drops of ink through extremely tiny nozzles. Bundled together, the hundreds of nozzles form a print head, which travels across the paper printing a horizontal line of the image. The nozzles fire many times per second. After completing a line, the paper is advanced and the next strip of the image is printed. This continues until the page is complete.
There are two basic types of inkjet printers: thermal and piezo. Most inkjet printers use thermal inkjet technology, which heats the ink to create a bubble that forces a drop of ink out of the nozzle. Tiny resistors may be used to rapidly heat a thin layer of liquid ink causing the bubble to form. As the nozzle cools and the bubble collapses, it creates a vacuum that draws more ink from a cartridge to replace the ink that was ejected. This process is repeated thousands of times per second. The time required to heat and then cool the nozzle theoretically slows printing speeds.
In contrast, piezoelectric inkjet printing, commonly referred to simply as piezo, pumps ink through nozzles using pressure. The print head regulates the ink by means of an electrical current passed through a material that swells in response to the electrical current to force ink onto the paper. Piezo print heads require vacuum pumps and large ink-absorbent pads to keep nozzles printing reliably. Piezo mechanical stability is also highly sensitive to small air bubbles, and the system may also need flushing with ink to purge trapped air, a process that wastes ink.
There are many causes of printing errors when using inkjet print heads. These problems mostly relate to a nozzle that is, for a variety of reasons, not functioning properly. For example, the expected drop of ink from a given nozzle may be misdirected or missing entirely due to manufacturing variations, material or geometry defects, resistor film defects, contamination, kogation, ink clogging, ink crystallization, nozzle plugging, etc. The result is an undersized, missing or misplaced dot on the print media. The print quality is consequently degraded and will be noticeably inferior to the human eye. If the output of the printer is to be optically scanned, photocopied or otherwise processed electronically, the defects will again be apparent.